In a gas turbine engine, hot combustion gases generally flow from one or more combustors through a transition piece and along a hot gas path. A number of turbine stages typically are disposed in series along the hot gas path so that the combustion gases flow through first-stage nozzles and buckets and subsequently through nozzles and buckets of later stages of the turbine. In this manner, the turbine buckets are subjected to high temperatures resulting from the combustion gases flowing along the hot gas path. Because the efficiency of a gas turbine engine is dependent on its operating temperatures, there is an ongoing demand for components positioned along the hot gas path, such as turbine buckets, to be capable of withstanding increasingly higher temperatures without failure or decrease in useful life.
Certain turbine buckets, particularly those of later turbine stages, may include a number of cooling holes extending radially through the turbine bucket. In this manner, the cooling holes may transport a cooling fluid, such as air, through the turbine bucket for exchanging heat in order to maintain the temperature of the turbine bucket within an acceptable range. According to one known cooling hole configuration, the turbine bucket may include a number of long, straight cooling holes formed by shaped-tube electrolytic machining, otherwise known as “STEM drilling.” Although such configuration may provide adequate cooling of the turbine bucket in certain applications, cooling holes formed by conventional STEM drilling are limited to a straight path through the turbine bucket. Accordingly, the three-dimensional shape of the turbine bucket also is limited due to the need to accommodate the straight cooling holes extending radially therethrough. Moreover, the straight cooling holes formed by STEM drilling have a constant diameter and thus fail to address the variation in cooling needs along the radial length of the turbine bucket. Specifically, as a result of the constant diameter, an undesirable amount of heat may be transferred to the cooling fluid before it reaches a tip region of the turbine bucket where cooling needs are greater.
There is thus a desire for an improved turbine bucket having a cooling configuration for withstanding high temperatures along the hot gas path of a gas turbine engine. Specifically, such a cooling configuration may allow the turbine bucket to have various complex three-dimensional shapes or twist for improved aerodynamics. Such a cooling configuration also may address the variation in cooling needs along the radial length of the turbine bucket for efficient cooling. Ultimately, such a cooling configuration may decrease the amount of air flow required to cool the turbine bucket while increasing overall efficiency of the gas turbine engine.